The assembly of printed circuits, and more specifically the permanent assembly and interconnection of packaged integrated circuit (IC) components and discrete electronic components (e.g., chip resistors, chip capacitors, diodes, etc.) to the outer surfaces of both rigid and flexible circuit boards, has involved the use of some form of solder alloy (e.g., Sn63Pb37) since the earliest days of the electronics industry. The reasons for using solder assembly are numerous, but perhaps most important is that it has cost effectively enabled the mass joining of thousands of electronics interconnections between printed circuits of every type including rigid, flexible, stretchable and various combinations of these structures with the electrical terminations of the myriad different types of electronic components which may populate their surfaces.
While solder alloys have been most common, over the history of the industry other joining materials have been proposed and/or used, including isotropic and anisotropic adhesives or so-called “polymer solders” which are a form of conductive adhesive. In some cases even non-conductive adhesives have also been suggested relying on capacitive coupling to make the electrical interconnection and carry the signal. Moreover, there have been efforts to make connections separable by providing sockets for components, to facilitate removal and replacement if needed or desired. In addition, there have also been electrical and electronic connectors developed to link power and signal carrying conductors with various resilient contact structures, however all such structures require constant applied force or pressure to maintain connection and they occasionally fail in non-benign environments.
Adhesive and socket solutions are attractive for some applications because they do not require the exposure of the components mounted on them to high temperatures and the potential damage associated with the temperatures required for soldering, however, each of these solutions also has limitations related to cost, performance, reliability and combinations thereof. Though improvements are constantly being made, current generation adhesives are not as conductive as electronic solder and sockets and while they allow for easy component removal and replacement, they also add unwanted weight, volume and expense to the final assembly.
In recent years the electronics industry has been forced by European Union legislation to eliminate the element lead (Pb) from solder, based on a presumption of risk to humans. This had had both economic and technical impact on the industry. For example lead free solders contain higher percentages of tin and commonly use silver which is very expensive. The net impact of this legislation has been deleterious to the electronics industry as electronic assemblies are potentially less reliable because of the higher temperatures required along with other reasons unanticipated at the time legislation was passed. For example, there is known to be an inverse relationship between temperature and the long term reliability of semiconductor devices. Others factor causing reduced reliability are the facts that the lead free solder joints are proving more susceptible to shorting due to tin whiskers due to the absence of lead in the solder alloy (as has been reported by NASA researchers) and more prone mechanical failure when accidentally dropped. Other deleterious phenomenon includes such defects as opens, shorts, pad cratering and solder joint cracking. In addition, the greater energy use required to achieve the higher temperatures associated with lead free soldering has a negative impact both on manufacturing cost and the environment. Present day technical and trade journals for the electronics industry are replete with articles and technical papers describing problems associated with lead-free soldering and research into ways to make the problem less onerous. The high temperatures of lead free soldering significantly reduce the number of options available for use as a prospective substrate and tend to force the user to employ more expensive substrate materials to address the challenge of the higher soldering temperatures required.
There are significant problems and many steps required for the manufacture of a traditional printed circuit assembly. The major steps are presented in FIG. 1 to appreciate the complexity. As can be seen in FIG. 2 if one eliminates soldering from the overall process the number of steps can be significantly reduced.
Given the aforementioned problems in the assembly of both rigid and flexible circuits related to solder, especially for lead-free solders. This topic has been discussed and solutions offered in a number of issued and pending patents including the following issued patents: U.S. Pat. No. 8,193,042 Flexible Circuit Assemblies without Solder and Methods for Their Manufacture, U.S. Pat. No. 8,093,712 Monolithic Molded Flexible Electronic Assemblies without Solder and Methods for Their Manufacture, U.S. Pat. No. 8,067,777 Light Emitting Diode Package Assembly, U.S. Pat. No. 7,981,703 Electronic Assemblies without Solder and Methods for Their Manufacture, U.S. Pat. No. 7,943,434 Monolithic Molded Flexible Electronic Assemblies without Solder and Methods for Their Manufacture and U.S. Pat. No. 7,926,173 Method of Making a Circuit Assembly, however there is room for further improvement in circuit manufacturing and assembly technology especially in the field of rigid flex circuit manufacturing technology.
For example, even if traditional solders could still be used there is a challenge facing the industry because of the faster operating speeds and thus higher operating temperature of present day higher performance electronic components. Optical transceivers are also troubled by high heat generation which must be controlled as the wave length of transmitted light can and normally will vary with change temperature. Thus components are more frequently requiring the use of metal (e.g. aluminum) and/or ceramic (e.g. aluminum nitride) materials as heat sinks and heat spreaders to help dissipate the higher thermal energy protecting the components from the damaging heat. In this regard is important to note that it is a general property of metals that they can conduct to varying degrees both electricity and heat, however, other materials, such as ceramics, can conduct heat but not electricity. While it would be ideal to make substrates out of metal or ceramic, the use of heat sinks further exacerbates the lead-free soldering problem as their conductive and large thermal capacity draws heat away from the lead free solder, extending thermal exposures to greater duration and damaging components even more and/or resulting in “cold” or incompletely formed solder joints. Given the aforementioned is used in the assembly of both rigid and flexible circuits with solder, especially for lead-free solders, there is room for further improvement in rigid flex circuit manufacture and assembly technology such as can be achieved by using solder alloy free electronic (SAFE) assembly methods